I have a 91 E-350 Club Wagon with 7.3 engine.
Here's the problem I'm trying to solve:


Head, tail, running, brake, dash, dome lights are not working
Horn not working
Cruise not working ( I can't recall if it ever did)


Radio works until you pull out the head light switch and the it goes to static and needs to be reset, the radio is all stock


Turn signals and hazards work
Back up lights work
The truck starts and runs great
Heater works
Wipers work


The fuses look good
I went through the test procedure for the head light switch and floor mounted dimmer switch and the check out ok


I can apply power to the headlight switch connector and all the lights work with power applied to each respective terminal


There is no power to the black/orange wire coming into or going out of the fuse block --- unless I pull the #8 fuse, then both wire are hot


Electrical is not my strong point any help is appreciated.

 

Here's a diagram.
http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=50734

You can see power on the black/orange comes from fusible link "F". You did a lot of troubleshooting already and that is helping. Your comment about pulling the fuse and then it's hot is a good clue that there is a corroded connection upstream from the fuse box.

Here's another diagram for out under the hood where the fusible links are located.

http://www.revbase.com/BBBMotor/Wd/DownloadPdf?id=11334

I can't seem to resolve the first diagram to the second, but in the first diagram it lists a splice "102". There is also a splice "102" in the second diagram. I do not see where the power in the second diagram goes to the headlamp switch (underhood fuse box you said?)

Anyway, I am sure your problem is going to be one of those splices or a corroded connection at the start solenoid. You can see in the second diagram, the starter solenoid is the take off point for many of those fusible links, they are the main power distribution paths for all the electrical. One of those splices or connections is corroded out there, that is why under load you get nothing, take the fuse out which takes the load off, and you get power. From your description it sounds like you are using a digital meter. This is fine except always test your circuits with them on. If you test with them off, you will get false readings like you got with the fuse pulled.

So turn the headlight switch on, put the fuse back in, and start poking around those wires in the engine compartment over at the starter solenoid. Or disconnect the batteries, and then take all the connections loose looking for corrosion and bad connections. The fusible links are the colored rubber things in the wires, pull on them. If they are stretchy they are bad.

 

Thanks for the feedback-I will report back with my findings.

 

OK, I was able to get out and finish the job this weekend.


I learned a lot from your post about the circuit failing under load and from a post you made in another thread regarding a test light. I bought a test light and started poking around.


There was power at the fuse box and light switch until I turned on the headlights and then it was immediately dead. Power would come back after a few moments. This confirmed the results I got when the fuse was removed.


I checked the power source at the solenoid and just downstream of fusible link F at the solenoid and there was good power under load with no sign of corrosion.


As with most vehicles, much of the harness is not readily accessible, so I started working my way away from the source and the light switch towards the middle. I finally ended up with a short section of the harness above the A/C pump behind the vacuum manifold. I pulled the vacuum manifold and there were 3 additional fusible links. One was crispy and hard. It was on the wire I was tracing. I replaced it and everything is working as it should now.


A couple of interesting things:


The size of the fusible links did not match the wiring diagram.


There was no splice 102 where indicated but near there is where I found the blown fusible link. I found the blown fusible link in another diagram listed as fusible link E, this diagram did not indicate the location or things would have progressed faster, or if the vacuum manifold wasn't hiding it.


Although the fusible link was compromised, it was still making some contact as the lights would work intermittently under light load. This is counter to what I though I knew about fusible links.


I recall the moment the circuit failed, I was heading home with my race car on the trailer and when I hit the high beams all the lights went out. I was able to continue with low beams after a cooling off period. I did not hit the highs again and at the time thought it was related to the dimmer switch. After reading other threads here regarding how poorly the lighting circuit is designed, I'm certain the failure was due to the extra load of the trailer, when I hit the highs beams it caused the overload.


Thanks Franklin2, I was able to make this repair with your help and I learned a lot from you for my next electrical gremlin. There's a beer in my fridge with your name on it if your ever traveling through the PNW.


Cheers, XSNRG